"someday your daughter will find out" is worded so that there is an implied threat but let's her off the hook and will allow her to play the victim because "I didn't say I would tell her! You are attacking me just like you do the SKs!" if you call her on it.

But it is pretty awful that she is gloating over the idea of your relationship with your daughter being damaged, if nothing else. The person that would hurt the most is her granddaughter. She would use her as a pawn to get back at you and not even blink an eye. 

If it was me, I would talk with my husband and craft a joint response that puts her on notice that she is not allowed to speak that way to you ever again. And that she will need to apologize to you if she wants to have a relationship with any member of your family going forward. And your DH is the person who delivers this message - soon.

If she throws another hissy fit, block her. You don't need this toxic person in your life. She's only doing it because she has been allowed to do so up until now. She won't stop her abuse toward you until you refuse to let her.

And the same goes for the SKs. You let them back in after they accused you of abuse which could have hurt your child. Of course they believe they can treat you anyway you want. You just have to stop letting them. Dad can choose to have a relationship going forward but they are adults and can meet up outside your home. And you don't have to let your child be part of it.

Your child deserves a mother who isn't stressed out from all these attacks. Children pick up on this and it affects them. And you deserve a life where you're not under attack from all these crazy people.
 

Before MIL can lie to your DD, get all of the ducks in a row and filed. Every fact, every bit of behavioral bullshit that your DD's elder shallow and polluted gene pool half spawn sibs have pulled. Every manipulation that MIL has attempted to play.  Have DH's divorce decree ready, the CO, the court hearing recordings, GrandHag MIL's hidden skeletons ready, and start informing your DD in an age appropriate manner as she grows up.  In the future when MIL decides that it is time to tell your DD the lies she will spout, your DD will already know the facts and the truth and will be able to call bullshit on Granny in real time when that time comes.  Toxic Skid butt sniffing HagGranny will melt into a puddle of butt hurt ectoplasm and she will deserve every bit of it when it happens.

It is sad to have to prepare kids for these eventualities but preparing them is far better than naively serving those kids up to a toxic player in their family dance.

When SS would ask questions upon return from a SPermLand visitation we answered them truthfully and with the facts.  We never lied to him, we did not manipulate. We informed and we showed him where that information came from and what the full view was rather than the randomly selected bitch and lie point that the SpermClan chose to play.  More specifically, the facts and an age appropriately informed SS gutted the campaign of control and PAS that SpermGranHag was hell bent on perpetrating.

SS:  "Dad, (SpermGrandHag says that  you are not my REAL dad and that you are only m StepDad."

Rags: "Son, your mom and I have talked with you about how a BioDad is the dad that made you with your mom and that a StepDad is the dad that is married to your mom and who loves you and your mom very much. A REAL dad is the dad who goes to work every day to make sure that we have a safe and nice home to live in, safe vehicles to travel in, good schools for you to go to, healthy food to eat, nice clothes and shoes to wear, coaches your sports teams, teaches you to read and write, to use the toilet, to tie your shoes, and ride your bike and who loves you and your mom very much.  A BioDad can be a REAL dad. A StepDad can be a REAL dad."

SS: "Dad, a StepDad sounds like a real dad to me. Can we go play?

Several years later.

SS: "Dad, (SpermGrandHag) says that you are not my dad and I can't call you dad. What should I call you?"

Rags: "Son, you started calling me Dad(dy) when you first started talking. No one every told you to call me Dad.  If Dad is no longer what you want to call me you can call me Mr. (LastName).  I am not your uncle and I only let your cousins call me Uncle (FirstName). I do not allow children to call me by my firstname. So, you can call me Mr. (LastName). (SpermGrandHag) does not decide what you call me.  You do, and I do."

SS: "I think I will stick with calling you Dad. That is what you are. You are my Dad."

Rags: Turning away to hide a tear....... "That works for me son."

Fuck the toxic opposition regardless of who they are. Your X, DH's X, your family, your ILs, and anyone else who is toxic and a detriment to your life, your family, and your children.  Prepare your DD in an age appropriate manner and she will skin and gut her idiot GM when necessary.

When they would ply their toxic manipulative lying bullshit with our son we countered with providing SS with the facts in response to whatever crap he dragged home with him after SpermLand visitation.  When he would then ask why they lied we would tell him that they are afraid and think that lying is better than the truth.  As he progressed through the 16+ years under the CO and then into his adult life not infrequently we found him digging through the Custody/Visitation/Support drawers in our home office file cabinets doing his own research. Occasionally he would bring us a document and ask us why one of the SpermClan lied about something that made no sense for them to lie about then he would show us in the documentation where it showed that they had lied to him.  Things like the SpermIdiot lying to his kids that he proudly had never being married when he had married a 16yo when he was 25 to avoid an arrest for statutory rape then divorced her a few months after the SpermClan got their asses handed to them in their attempt to take custody of SS away from my DW.  A copy of his marriage license and divorce decree wa in our files.  Lies about how CS paid for expensive things for our family. New homes, nice cars, nice clothes, top schools for SS, vacations, etc....  "Mom and Dad, how does $133/mo in CS pay for all of these things when it clearly pays for nothing."  He was holding the CO and a spreadsheet with colorful charts showing CS in reference to our income and expenses that his CPA mom had put together for him the last time that lie caused our son confusion.  The message was don't lie to a kid who has the full facts the entire truth, and access to every tiny little bit of relevant information that influences their own blended family life.

These are just a few of the related Q&A fact review sessions we had with SS as he was growing up and dealing with the shallow and polluted end of his gene pool.

I have had a few career offers in SpermLand over the 30 years of our blended family adventure.  SS has been firm and clear that he does not support us ever living there and when I predecease his mom that she can't move there.  He knows the shit show that SpermLand is. Sadly not only related to the SpermClan but also related to my DW's branch of SS's three legged stool of a family. He loves them and they are orders of magnitude superior to the SpermClan though their drama is nothing that SS wants to have much to do with either and does not want his mom living the hearbreak that he own family is.

Kids need the facts. MIL will not stop her shit when your DD reaches adulthood and the risk is always present that your DD's elder half sibs will take a run at demonizing you and her daddy to your DD.  Facts, discussion, Q&A, CO reviews, Supplemental rules, State regulations, etc... in an age appropriate manner rips the guts out of the toxic opposition when a kid is confident, informed, and armed with the facts to in real time counter any toxic crap.

Be cognizant of the age appropriate part. Though sometimes the toxic opposition will force an earlier than we would like need to explain the facts to a confused kid.

IMHO of course.

Good luck.